Formation of epoxy-diamine/metal interphases

Abstract

Epoxy-diamine networks are extensively used as adhesives or paints in many industrial applications. When the precursors are applied onto metallic substrates and cured, an interphase, having chemical, physical and mechanical properties quite different from that of bulk polymer, is created between the substrate and the polymer. Moreover, chemical reactions between diamine and metallic surfaces induce an increase in the practical adhesion (adherence). When the same epoxy-diamine mixtures are applied onto gold coated or polyethylene substrates, the interphase properties are the same as bulk ones. When epoxy-diamine mixtures are applied onto aluminum or titanium alloy surfaces, the glass transition temperature, amine and epoxy reaction extent, the interphase thickness, residual stresses within the interphase, Young's modulus of the interphase all depend on the amine nature (aromatic, aliphatic or cycloaliphatic), the stoichiometric ratio, the processing conditions (time and temperature), the organic layer thickness and the metallic surface treatment. Coating analyses (FTIR, FTNIR, DSC, DMTA, H + and C 13 NMR, SEC, ICP and POM) suggest that diamine monomers chemically react with and dissolve the metallic hydrated oxide layer. Then, metallic ions diffuse through the organic layer to form a complex by coordination with diamine monomers (chelate or ligand). Metal–diamine complexes are insoluble, at room temperature, both in diamine as well as in DGEBA monomers and they induce a phase separation during the curing cycle of the epoxy-diamine precursors. Furthermore, the chemical bonding of diamine monomers to the metallic surfaces and the diamine–metal crystal orientation parallel to the metallic surface within the interphase lead to chemical, physical and mechanical properties to the epoxy-diamine network which are different from those of the bulk.